The invention relates to an arrangement comprising a semiconductor laser diode in which one of the end faces is adapted to be coupled to an optical transmission waveguide.
When a semiconductor laser diode having a large coherence length is coupled to a glass fiber having a graded refractive index profile or, in general, to a multimode fiber and if there are one or more non-ideal fiber couplings or, in general, connections having a spatial filter action in the transmission path, then what is commonly known as modal noise is produced at the receiving end of the fiber.
The modal noise phenomenon is described in the Proceedings of the Fourth European Conference on Optical Communication, Sept. 12-15, 1978, Genova, pages 492-501. It is also mentioned there that the use of a laser which has a larger spectral line width of one longitudinal mode (and consequently a smaller coherence length) can greatly reduce the modal noise.